Analytical and diagnostic determinations are frequently performed on liquid samples, comprising in addition to the analyte of interest, also countless other components, in solution and/or in particulate form, which often interfere with the handling of the sample and may influence the quantitative or qualitative determination of the analyte.
For example, numerous clinical diagnostic methods are based on the detection of an analyte in a biological sample. Frequently, such detection is achieved in a disposable assay device, allowing rapid and simple diagnosis. One important application is the wide field of immunology, where analytes are detected with the aid of specific antibodies, capable of binding to the analytes and forming detectable complexes, usually with the aid of ligands aiding the detection.
When performing a test using a biological sample from a patient, in particular a blood sample, many factors need to be considered. Whole blood is prone to clotting, reducing or preventing the desired flow of the sample in the assay device. The red blood cells, even in the absence of clotting, may inhibit or retard flow. Further, red blood cells may inhibit binding between specific binding pair members. Red blood cells also have enzymatic activity, which, depending on the assay employed, may interfere with the signal produced.
Unfortunately, red blood cells present in whole blood also scatter and absorb light thus interfering with assay methodologies which measure either reflected or transmitted light. Also other cells may interfere with particular determinations; for example, cholesterol determinations can be effected by cholesterol present in cell membranes.
Further, the red cell fraction takes up a considerable volume of the sample, in some cases as much as half the volume. Importantly, this fraction, also called hematocrit, may vary between different individuals and even in the same individual, between different measurements. This in turn may influence the accuracy and/or the repeatability of the determinations.
Consequently many assays involve a step of separating the red blood cells, whereupon the assay is carried out on plasma or serum. When the separation is performed before clotting, plasma is obtained. When clotting has occurred before separation, serum is obtained.
The red blood cells can be separated from plasma through centrifugation, which however requires a relatively large volume of sample, and the use of a centrifuge. This is also time consuming and constitutes an additional step of handling the sample, which increases cost and complexity, and which further should be avoided in particular when potentially contagious blood-borne pathogens are involved. Additionally, the risk of the sample being contaminated by the individuals handling it, cross-contaminated by parallel samples or mixed up with other samples is increased.
What is said above regarding whole blood samples and red blood cells applies also, with necessary adaptations, to all other biological samples, where cells, cell debris, fibres, or other unwanted particles etc., may interfere with the determination and should therefore preferably be separated before or during the reaction or determination leading to the detection of the analyte.
The most common type of disposable assay device consists of a zone or area for receiving the sample, a reaction zone, and optionally a transport or incubation zone connecting the receiving and reaction zone, respectively. These assay devices are known as chromatography assay devices or simply referred to as strip tests. They employ a porous material defining a path for fluid flow capable of supporting capillary flow, e.g. a filter material. The sample-receiving zone frequently consists of a more porous material, capable of absorbing the sample, and, when the separation of blood cells is desired, effective to trap the red blood cells. Examples of such materials are fibrous materials, such as paper, fleece, gel or tissue, comprised e.g. of cellulose, wool, glass fibre, asbestos, synthetic fibres, polymers, etc. or mixtures of the same. The transport or incubation zone commonly consists of the same or similar materials, often with another porosity than the sample-receiving zone. Likewise, the reaction zone, which may be integrated with the incubation zone, or constituting the most distal part thereof, commonly consists of similar, absorbing fibrous materials, or any of the above listed materials.
In an assay device or strip test, the porous material(-s) is (are) assembled on a carrier, such as a strip of thermoplastic material, paper, cardboard or the like. Further, a cover can be provided, said cover having at least one aperture for receiving the sample, and an aperture or transparent area for reading the result of the assay.
Nitrocellulose materials are also frequently used as the matrix constituting the transport or reaction zone, connecting the receiving zone and the reaction zone. A significant disadvantage with nitrocellulose is its high non-specific binding of proteins and other bio-molecules. Present test strips however often handle a surplus of sample, reducing the influence of this binding. It is however desirable to minimise the sample volume, in line with the tendency to miniaturize the entire test, including minimising the amounts of reagents, without compromising accuracy and reliability.